Methanol and dimethyl ether (DME) are known for use as fuels for solid oxide fuel cells (SOFC). They could be attractive fuels for use in SOFC combined heat and power plants, for instance those plants intended for use as auxiliary power units for marine applications. Potentially the fuel processing steps in such a plant could be very simple ultimately being only evaporation of the methanol or DME and injection into the anode chamber of the SOFC.
This approach would, however, lead to a number of problems and disadvantages:
Saunders, G. J. et al. (Formulating liquid hydrocarbon fuels for SOFCs, Pages 23-26, from Journal of Power Sources Volume 131, Issues 1-2, Pages 1-367 (14 May 2004)) mentioned that dry methanol was prone to form carbon at conditions prevailing in the anode chamber of the SOFC with the most active Ni-cermets as anode material. The results of Saunders et al. showed that only two liquids, methanol and methanoic acid could be injected directly onto nickel cermet anodes without serious carbon blockage. Even then, small amounts of carbon deposition were revealed which could be prevented by adding low amounts of air or water to the fuel.
Carbon formation in a SOFC plant can take place by the following reversible reactions:CH4C+2H2 (−ΔH298=−74.9 kJ/mol)  [1]2COC+CO2 (−ΔH298=172.4 kJ/mol)  [2]
Reaction [2] is known as the Boudouard reaction. Both methanol and DME can decompose to form CO according to reactions [3] and [4]:CH3OHCO+2H2 (−ΔH298=−90.7 kJ/mol)  [3]CH3OCH3CH4+CO+H2 (−ΔH298=1.3 kJ/mol)  [4]
As CO is quite reactive, it is important to know the temperature and gas composition ranges, where reaction [2] does not occur. This can be studied using “the principle of the equilibrated gas” assuming both methanation/steam reforming (reaction [5]) and the shift reaction (reaction [6]) to be in equilibrium, as further described by Nielsen, J. R. (Catalytic Steam Reforming, Springer Verlag, Berlin 1984).CH4+2H2OCO2+4H2 (−ΔH298=−165.0 kJ/mol)  [5]CO+H2OCO2+H2 (−ΔH298=41.2 kJ/mol)  [6]
Sasaki, K. and Teraoka, Y. (Equilibria in Fuel Cell Gases Pages 1225-1239 from Solid Oxide Fuel Cells VIII (SOFC VIII) Proceedings Volume 2003-07) have studied the amount of water needed to avoid carbon formation.
The direct use of DME in SOFCs has also been reported in the literature by Dokiya, M. et al. (Partial Oxidation Reforming of Dry Diesel Oil, Dimethyl-Ether and Methane using SOFC, pages 1260-1265, from Solid Oxide Fuel Cells VIII (SOFC VIII) Proceedings Volume 2003-07, The Electrochemical Society) and by Tatemi, A. et al. (Power Generating Property of Direct Dimethyl Ether SOFC using LaGaO3− based Perovskite Electrolyte, pages 1266-1275 from Solid Oxide Fuel Cells VIII (SOFC VIII) Proceedings Volume 2003-07, The Electrochemical Society). One disadvantage was that the open circuit voltages obtained were considerably lower than those obtained using hydrogen as fuel for the SOFC. It was, however, mentioned that only minor amounts of carbon were observed in the short term test stated. There was no mention of the means used to preheat DME to anode operating temperatures in excess of 600° C.
From our knowledge, in an industrial facility such preheat would have to take place in an in/out heat exchanger, which most cost effectively and conveniently would be made of steel. Such heat exchangers would be very prone to carbon formation and metal dusting, if dry methanol or DME were used as feed for the SOFC.
A further disadvantage of using methanol or DME compared to using methane is related to the heat of reactions when steam reforming these fuels. Steam reforming of methane is given in equation 5 and the reforming reactions for methanol and DME are given in equations 7 and 8, respectively:CH4+2H2OCO2+4H2 (−ΔH1023=−191.4 kJ/mol)  [5]CH3OH+H2OCO2+3H2 (−ΔH1023=−70.3 kJ/mol)  [7]CH3OCH3+3H2O2CO2+6H2 (−ΔH1023=−160.0 kJ/mol)  [8]
Reforming of the fuel in the anode chamber (internal reforming) helps to cool the stack due to the endothermal nature of the reforming process. However, the heat of reactions for methanol and DME reforming are much less endothermic than methane steam reforming, therefore the cooling of the stack provided by steam reforming of methanol or DME is less effective.
The fuel processing method of the invention describes a process lay-out where all the above problems are overcome by adiabatically converting methanol or DME into a mixture of methane, CO, CO2 and water.
It is an objective of the invention to provide a fuel processing method for solid oxide fuel cells, whereby the fuels methanol and DME are adiabatically converted to a mixture of methane, CO, CO2 and water before conversion in a solid oxide fuel cell.